1. Field of the Invention
The present invention relates to an ettringite binder for a mortar, including for a dense mortar, preferably with a water/solid weight ratio lower than 0.5, said binder comprising a calcium aluminate and calcium sulphate mineral compound.
It is meant under ettringite binder a hydraulic binder with the components thereof giving, upon hydration under normal use conditions, as the main hydrate ettringite, which is a calcium trisulphoaluminate having as a formula 3CaO,Al2O3.3CaSO4.32H2O.
It is meant under solids all the dry components of the mortar.
The invention also relates to a dry mortar formulated from such an ettringite binder comprising the calcium aluminate and calcium sulphate mineral compound.
The invention further relates to a wet mortar obtained from mixing dry mortar such as defined hereinabove, with water in such an amount that the water/solid weight ratio is lower than 0.5.
Finally, the invention relates to the use of a calcium aluminate mineral compound for formulating an ettringite binder, a dry mortar or a wet mortar such as defined hereinabove.
The ettringite binder comprising a calcium aluminate and calcium sulphate mineral compound is adapted to be used in mortars and concrete in a building where a quick reoperation of the structure is expected. More particularly, it makes it possible to make floor repair and preparation products such as, for example, screeds, smoothing coatings, paving glues.
A quick reoperation for structures requires reaching, depending on the applications, a minimum mechanical strength level at a given time and/or a coating time defined by the residual moisture in the material. The products adapted for a quick reoperation are conventionally prepared from a binder with the hydration thereof leading to forming ettringite.
2. Description of the Related Art
In the smoothing coating application for example, according to the specifications of the Centre Scientifique et Technique du Bâtiment “Produits et systèmes de préparation de sols intérieurs pour la pose de revêtements de sols minces”—Guide technique pour l'avis technique et le classement P. Cahiers du CSTB, no2893 —Delivery no370, Jun. 1996), products should meet at the same time mechanical performance criteria, adhesion performance criteria as well as use usability criteria [paste homogeneity, fluidity (spreading diameter of the paste being previously poured into a ring with 30 mm height and 50 mm diameter) and gelling time].
In addition to the criteria as required by the CSTB, the quick smoothing coatings should at least meet the following criteria under normal temperature and hygrometry conditions:                spreading of 150 mm at 7 and 20 minutes;        mechanical compression strengths higher than 4 MPa at 4 hours;        24 h coating time (3% residual moisture in the material for coating thicknesses lower than 10 mm); and        mechanical compression strengths higher than 25 MPa at 28 hours.        
The ettringite-forming chemical reaction is as follows:6Ca2++2Al(OH)4−+3SO42−+40HO−+26H2O→3CaOAl2O3.3CaSO4.32H2O
The ettringite solubility product at equilibrium is: Kett32 4,9×10−44. The formation rate of ettringite (nucleation degree and ettringite crystal growth) depends on several parameters, including the oversaturation coefficient β, linked to the energy available for forming nuclei:β=(aCa2+)6*(aAl(OH)4−)2*(aSO42−)3*(aOH−)4/Kettwherein ai stands for the activities of ions i.
Ettringite can be obtained from the hydration of compositions comprising calcium aluminates and a source of sulphate, and optionally Portland cement and/or lime bringing in solution the ions required to perform such a chemical reaction. Calcium aluminates are combinations of aluminium oxide Al2O3 represented by A in the cement nomenclature, and of calcium oxide CaO represented by C in the above mentioned cement nomenclature, such oxides being crystallized more particularly under the C3A, C12A7 and CA forms.
In practice, formulations for mortars with quick hardening and drying nowadays comprise an association of calcium aluminates, calcium sulphate and Portland cement, with the proportions of each of the components being tricky to define, as the hydration under the ettringite form should be controlled so as to reach the best compromise between the ettringite amount being that ensures the drying ability of the product (high amount of mixing water being crystallized under the form of hydrates) and the morphology of such an ettringite, which, for a given crystal density, ensures the mechanical strength level and the control of the dimensional variations throughout the entire hardening process up to the long term. Such a compromise is all the more harder to reach as the fastness levels of strength acquiring to be obtained should be compatible with the expected implementing characteristics, in particular the workability hold time.
Such a compromise is not obtained satisfactorily in mortars of the prior art.
Thus, for example, U.S. Pat. No. 4,350,533 discloses ettringite cement compositions based on calcium aluminate, calcium sulphate cements, more particularly under the form of gypsum, and optionally separately supplied lime and Portland cement. But the development kinetics for mechanical strengths is much lower than the one desired within the scope of the present invention.
It is known, for so-called “Mine Packing” applications (where it is aimed at filling the recesses occurring in underground structures), to use ettringite mixtures of calcium aluminates and calcium sulphate. But the requirements of the system differ considerably from the “dense mortar” applications of the invention: the product should be pumpable, set quickly, but with a water/solid ratio in the order of 5 (the important point in such an application being to create much volume), the mechanical compression strengths at 24 hours do not exceed 5 MPa. Further, the system durability is not a key criterium, neither are the dimensional variations. The existing requirements in “dense mortar” applications are such that it is not possible to use such “mine packing” solutions directly, they are to be reformulated and adapted to the dense system requirements.